It is known to use ultrasonic inspection apparatus for non-destructive testing or checking of materials and in medical echography. In the field of materials testing, these techniques enable the detection of heterogeneity and defects in the parts under test, as well as the characterization of the defects.
The transducers employed in these ultrasonic inspection devices are designed to generate sound waves from electrical energy which is supplied to them, and the sound waves are propagated in the medium to be tested for reflection by reflectors. The waves thus reflected are received either by the transmitting transducer acting also as a receiver, or by another transducer acting only as a receiver, and are converted by the receiving transducer into electrical signals which are then analyzed.
The life of these transducers is limited, and replacing them with transducers having identical theoretical characteristics should not put the results of the measurements in doubt. However, it has been found that substantial variations can be obtained between the results of tests carried out in an identical manner with different transducers having identical theroetical characteristics, even after calibration. These variations relate in particular to the amplitude of the fault echos, which could vary by up to a factor of 2.
J. L. Pelletier's work: "La Pratique du Controle Industriel par Ultrasons", Volume 2, published by Editions Communications Actives, points out on page 55 that numerous difficulties and errors originate from the transducers and that it is therefore necessary to know their characteristics. The book describes several characteristics to be determined, particularly the operating frequency. The measurement of this frequency is taken by spectral analysis of the echo obtained from a water/air interface of dimensions which can be regarded as infinite, the beam of the ultrasonic waves emitted being perpendicular to the interface, and the interface being situated at the limit of the near field for plane transducers, or at the focal point for focussed transducers.
In spite of the determination of these characteristics, however, the problem of reliability and reproducibility of ultrasonic inspections has not been solved. The aim of the present invention is to solve this problem by providing a method of selecting transducers for use in ultrasonic inspection apparatus such that substantial uniformity of performance can be assured.